Using crystal violet binding assay, we examined the potential for biofilm formation in 194, 76 and 107 isolates from urine of patients with uncomplicated acute cystitis, pyelonephritis and prostatitis, respectively. The prostatitis isolates showed significantly higher optical density (OD) values compared with cystitis and pyelonephritis isolates (OD(540): 0.82, 0.29 and 0.43, respectively, P<0.0001). Similarly, strains of serotypes O4 and O22, which were commonly isolated from prostatitis, exhibited significantly higher OD values than did other strains. Furthermore, when the 21 prostatitis isolates were examined for expression of curli fimbriae, eight of 12 strains with a high OD value, but only three of nine with a low OD value, expressed curli fimbriae (P=0.0195). These results suggest an association between acute bacterial prostatitis and biofilm formation.